Orthogonal frequency division multiplexing (OFDM) technology was firstly used in military field in 1960s, the modulating mode has very high frequency spectrum utilization efficiency and is suitable for wireless data transmission, so it has been used widely as a wireless access method.
OFDM system adopts multi-carrier technique, and a high speed data stream is transmitted by serial to parallel convert, so that the duration of data symbols in each sub-carrier increases relatively, and then the ISI (InterSymbol Interference) brought by the time dispersion of wireless channel could be reduced efficiently, thereby reducing the complexity of balance in a receiver, and even avoiding using an equalizer and only using the method of inserting cyclic prefix to eliminate the unfavorable influence of ISI. Orthogonality exists among the sub-carriers of the OFDM system, which permits mutual overlap of the frequency spectrum of each sub-carrier, and therefore, compared with the conventional frequency division multiplexing system, the OFDM system could maximize the use of frequency spectrum resources. FIG. 1 is a time domain wave shape view of an OFDM symbol in which the prefix is inserted cyclically.
In FIG. 1, Tb represents the effective symbol cycle in OFDM signals, Tg represents the inserted cyclic prefix, and the contents of Tg is the copy of the last part of contents in Tb period, and Ts is the cycle of the whole OFDM symbols. As long as the length of the cyclic prefix is longer than the maximum time delay extension of OFDM symbols, the orthogonality of each sub-carrier in one OFDM symbol can be guaranteed. And therefore, the multi-path interference could be overcome through inserting the cyclic prefix Tg periodically. It is usually required that the cyclic prefix should be bigger than or equal to 2 to 4 times of the maximum time delay extension when design an OFDM system, and table 1 shows the maximum time delay extension in common environments.
TABLE 1environmentmaximum time delay extensionIndoor40 ns~200 nsoutdoor1 μs~20 μs
It can be seen from table 1 that if the OFDM system only needs to support the wireless communication in indoor environment, usually the cyclic prefix should be bigger than 800 ns, whereas if the outdoor wireless communication needs to be supported, the cyclic prefix should be bigger than 20 μs, and because of the limitation of the time delay extension, in the condition that the FFT dots are fixed, the OFDM system should not adopt a too wide bandwidth.
In a broad sense, the OFDM system comprises OFDM and OFDMA systems, and OFDMA has the similar basic principle to that of the OFDM, while the difference of the two lies in that the bandwidth assignment of the OFDMA system is carried out in the two-dimensional interval of time and frequency, and the bandwidth of the OFDM system is only assigned on time dimension. Compared with the conventional OFDM system, the bandwidth assignment of the OFDMA system is more flexible and suitable for use in cellular networking, so it is attracting people's attention more and more.
The IEEE802.16 standard adopts OFDM and OFDMA techniques, but the two kinds of techniques are independent of each other relatively in the standard, and there is not any special definite provision on cooperative work of the two systems. From the point of compatibility, research on the coexistence system of OFDM and OFDMA is needed so that the future system could support OFDM and OFDMA techniques at the same time, and it could furthest protect the interests of investors and users.
Furthermore, to implement the coexistence of OFDM and OFDMA, it is needed to consider not only the compatibility of the frame structure to different systems, but also the limitation of cyclic prefix on the bandwidth of OFDM system and OFDMA system.
The calculating formula of the cyclic prefix in OFDM symbols is as follows:
            T      g        =                  G        ·                  T          b                    =                        G          ·                      N            FFT                                                n            ·            BW                    ⁢                                                      ,
wherein n is an over-sampling factor, and as for the OFDM system, the value of n relates to the bandwidth, and its range is basically between 1.14 to 1.15, but to an OFDMA system, the value of n generally is 8/7. And G is the cyclic prefix factor, which has 4 values as follows: ¼, ⅛, 1/16, and 1/32. NFFT is the number of FFT dots.
From the formula, it can be seen that the length of the cyclic prefix is in direct proportion to G, is in inverse proportion to the bandwidth, and is in direct proportion to the number of the FFT dots. Generally speaking, the number of the FFT dots adopted by the OFDM system is smaller than that of the OFDMA system, and the OFDM system generally adopts the FFT of 256 dots, while the OFDMA system adopts the FFT of 2048 dots. To the OFDM system of 256 dots, because of the limitation of the cyclic prefix, the bandwidth can not be too wide, but to the OFDMA system of 2048 dots, in the condition of the same time delay extension, as the number of the FFT dots is 8 times as that of the OFDM system, the largest bandwidth that the OFDMA system could adopt also is 8 times as that of the OFDM system. When the OFDMA system carries out cellular networking, the bandwidth should not be too small in order to guarantee the capacity of the system. Table 2 shows the range of the cyclic prefixes of OFDM of 256 dots and OFDMA of 2048 dots in every usable bandwidth condition as follows:
TABLE 2Minimum cyclicMaximum cyclicFFT dotsbandwidthprefixprefix25610MHz700ns5.6μs2567MHz1μs8.02μs2563.5MHz2μs16.04μs2561.75MHz4μs32.08μs2565.5MHz1.45μs11.64μs2563MHz2.67μs21.3μs204820MHz1.4μs11.2μs204810MHz2.8μs22.4μs204828MHz2μs16.04μs204814MHz4μs32.08μs20487MHz8μs64.16μs20483.5MHz16μs128.32μs204817.5MHz3.2μs25.66μs20488.75MHz6.4μs51.32μs20481.25MHz35.6μs284.44μs
From table 2, it can be seen that if it is desired to ensure the bandwidths of the OFDM system and the OFDMA system are the same, because of the impact of the maximum time delay extension, the bandwidth should not be too high, and if the capacity of OFDMA system is taken into consideration, the bandwidth that the OFDMA system adopts should not be too low, either. Owing to the above two reasons, the bandwidth which the OFDMA system adopts may be different from that of the OFDMA system. When the bandwidths of the two systems are remarkably different from each other, to the OFDM and OFDMA time division combined frame fashion in the conventional time division fashion, the utilization rate of frequency spectrum will be very low.
Therefore, in order to improve the utilization efficiency of the frequency spectrum of the OFDM and OFDMA coexistence system, it is necessary to design an efficient method for assigning the frequency spectrum bandwidth of the system.